Winglet

ABSTRACT

A winglet comprising a tubular main spar with forward and aft main spar webs joined by upper and lower main spar caps; a front spar with a front spar web, an upper front spar cap, and a lower front spar cap; an upper skin joined to the upper main spar cap and the upper front spar cap; and a lower skin joined to the lower main spar cap and the lower front spar cap. The winglet is manufactured by co-curing the upper skin to the upper main spar cap and the upper front spar cap; and co-curing the lower skin to the lower main spar cap and the lower front spar cap.

FIELD OF THE INVENTION

The present invention relates to a winglet, an aircraft wing comprisinga winglet, and a method of manufacturing a winglet.

BACKGROUND OF THE INVENTION

In the large civil aircraft aviation industry the growth in size of wingtip devices over the years as a result of drive to increase wingefficiency through reduction of drag has lead to technical challengesrelated to the load transfer and efficient joint technology between thewing tip device and wing. Existing large civil aircraft wing tipattachment methods, such as that described in U.S. Pat. No. 7,975,965B2, are generally made up of a ‘back-to-back’ rib solution where theloads are transferred across a joint utilising the chord depth of thelocal wing section.

An innovative solution created to decouple the limitations of loadtransfer through local chord depth is described in US 2012/0112005 A1.This idea proposes a joint concept that utilises a ‘main beam’ structureto carry the primary wing tip loads and transfer these into the wing viaan increased moment arm.

However, the wing tip device tends to be over-engineered, particularlyat the attachment point, in order to guarantee the mechanical propertiesrequired for the use of such fastening means because currentmanufacturing methodologies make it difficult to adequately tailor thestructural behaviour of the composite beam.

It remains difficult to manufacture and construct using compositematerials the complex spar geometry that enables a winglet to beattached to a main wing element. The use of conventional methods such asan assembly of multiple parts to form the spar are difficult due to thelack of access in the geometry available for tooling and assembly, andalso inefficient as a result of requiring an increased number of parts,thus increasing cost and weight of the final component, or resulting ina compromise of the structural design to meet the manufacturingconstraints.

A known braiding process for forming a complex shaped fibre preform isdescribed in U.S. Pat. No. 8,061,253. The method comprises braiding aplurality of fibres over a non-cylindrical mandrel to form a variablethickness shaped fibre preform. The preform is subsequently flattenedand cut to form the spar component. The mandrel is moved at a constantspeed during the braiding process.

As noted in J. S. Tate, A. D. Kelkar, and V. A. Kelkar, “Failureanalysis of biaxial braided composites under fatigue loading”, The15^(th) European Conference of Fracture (ECF), Stockholm, Sweden, Aug.11-13, 2004, when a biaxial braid tube is used for a component ofvarying cross-section, the braid angle, thickness and areal weight(yield) vary from point to point.

White, Mark L. Development of Manufacturing Technology for Fabricationof a Composite Helicopter Main Rotor Spar by Tubular Braiding. Vol.1618. KAMAN AEROSPACE CORP BLOOMFIELD CT, 1981 describes a braided sparfor a helicopter main rotor. Each braided layer is designed to beapplied at a constant pitch (i.e., mandrel advance per revolution of thebraider carriers) allowing the fibre orientation angle to decrease andthe layer thickness to increase as circumference decreases along thetapered spar.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a winglet comprising a tubularmain spar with forward and aft main spar webs joined by upper and lowermain spar caps; a front spar with a front spar web, an upper front sparcap, and a lower front spar cap; an upper skin joined to the upper mainspar cap and the upper front spar cap; and a lower skin joined to thelower main spar cap and the lower front spar cap.

The winglet can be attached to the tip of the main wing element of anaircraft wing, and the spar may be canted (up or down) and/or swept(forward or aft) relative to the main wing element. Typically thetubular main spar of the winglet has an inboard portion, and an outboardportion which is canted (up or down) and/or swept (forward or aft)relative to the inboard portion. Typically the main wing elementcomprises a rear spar which is attached to the tubular main spar of thewinglet (typically by one or more fasteners such as bolts which passthrough the two spars); and a front spar which is attached to the frontspar of the winglet (also by one or more fasteners such as bolts whichpass through the two spars). The wing may be a fixed wing (fixed to anaircraft fuselage) or a rotary wing (for a helicopter or other rotarywing aircraft).

The upper and lower front spar caps may extend aft towards the mainspar. However a problem with such an arrangement is that the upper andlower skins must be formed with joggles to enable a leading edge skinassembly to be attached to them. Therefore more preferably the upperfront spar cap extends forwards away from the main spar and the lowerfront spar cap extends forwards away from the main spar. Such forwardlyextending spar caps are preferred since they enable a leading edge skinto be attached directly to the spar caps without having to form jogglesin the skins.

The front spar may be tubular with forward and aft front spar websjoined by the upper and lower front spar caps. Alternatively the frontspar may be C-shaped with the upper and lower front spar capsterminating at forward edges.

The skins may be joined to the spars by fasteners but more preferablythey are bonded to the spars by co-curing, co-bonding or secondarybonding.

A leading edge skin may be joined to the upper and lower front spar capsby fasteners, or bonded by co-curing, co-bonding or secondary bonding.

A further aspect of the invention provides a method of manufacturing thewinglet of the first aspect, the method comprising co-curing the upperskin to the upper main spar cap and the upper front spar cap; andco-curing the lower skin to the lower main spar cap and the lower frontspar cap.

During the co-curing process the webs and caps of the main spar may becompacted against a first tool inside the main spar. Similarly the upperskin, the lower skin, the forward main spar web, and the front spar webmay be compacted against a second tool between the main spar and thefront spar. Similarly the front spar web and the upper and lower frontspar caps may be compacted against a third tool in front of the frontspar web. The tools may be removed after the co-curing or may be left inthe finished article.

Optionally the tubular main spar comprises a braided composite spar. Thebraided composite spar may comprise a plurality of tubular plies ofbraided fibres, each ply comprises a first set of fibres which wind in aclockwise direction in a first series of turns with a pitch between eachadjacent pair of turns, and a second set of fibres which wind in ananti-clockwise direction in a second series of turns with a pitchbetween each adjacent pair of turns, the first and second sets of fibresin each ply being intertwined to form a braided structure; wherein thespar extends lengthwise from a root to a tip, the spar has a taperedportion which tapers inwardly towards the tip, each ply has acircumference in the tapered portion which reduces as it tapersinwardly, and for at least one of the plies the pitches of the first andsecond sets of fibres increase as the ply tapers inwardly in the taperedportion. This enables fibre angles to be varied within the spar withouta step change in the plies and without stopping the formation process.

There are two benefits in increasing the fibre pitch (by increasing theratios S/ω1 and S/ω2) in the direction of inward taper and reducingcircumference. Firstly it causes a reduction in fibre angle which goesbeyond that which would be created by winding the fibres onto themandrel at a constant speed and pitch (as in the prior art). Thisenables the structural properties of the spar to be tailored asrequired—for instance providing higher bending stiffness at the tip thanat the root. For example the first and second sets of fibres may have afibre angle which changes by more than 10° or 15° in the taperedportion. At the same time it counteracts the tendency of the taperingmandrel to gradually increase the areal weight and thickness of eachply. Thus each ply may have an areal weight or thickness which does notchange in the tapered portion, or at least does not change by more than10% or 5% within the tapered portion. Typically each ply also has anareal weight or thickness which does not change by more than 10% or 5%over the entire length of the spar or preform. Providing a relativelyconstant areal weight and/or thickness (despite the tapered shape of thespar or preform) enables the spar to be modelled and analysed moreeasily by computer-aided design.

The tapered portion of the tubular main spar may extend over its fulllength from its root to its tip. Alternatively the tubular main spar hasan inboard portion (which may be non-tapered) between the taperedportion and the root. One or more fastener holes may be provided in theinboard portion. The tubular main spar may have an outboard portion(which may be non-tapered) between the tapered portion and the tip.

Optionally the tubular main spar has a centre line which extendslengthwise from a root to a tip and at least part of the centre linefollows a curved path which does not lie in a single plane.

Optionally the tubular main spar comprises a plurality of tubular pliesof braided fibres, wherein the tubular main spar extends lengthwise froma root to a tip and has a tapered portion in which each ply has a heightwhich reduces and a width which increases as it extends toward the tip.In a conventional braided spar of varying circumference, the fibreangle, thickness and areal weight vary as the circumference varies. Thisspecial shape has a particular benefit since it enables the height ofthe spar to be reduced without a large accompanying change incircumference.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 a is front view of an aircraft;

FIG. 1 b is a plan view of the aircraft;

FIG. 1 c shows a winglet installation at the tip of the port wing;

FIG. 2 is an isometric view of the main spar beam of the winglet;

FIG. 3 a is a front view of the spar of FIG. 2;

FIG. 3 b is a plan view of the spar of FIG. 2;

FIG. 3 c is a side view of the spar of FIG. 2;

FIG. 4 a is an isometric view of the spar of FIG. 2 denoting referencepoints for sectional views;

FIG. 4 b is a sectional view of the spar of FIG. 4 a at section A-A andB-B;

FIG. 4 c is a sectional view of the spar of FIG. 4 a at section C-C;

FIG. 4 d is a sectional view of the spar of FIG. 4 a at section D-D;

FIG. 5 is a schematic diagram of a braiding apparatus;

FIG. 6 is an end view of the bobbin braiding ring;

FIG. 7 is a schematic view of the mandrel showing the change in fibrepitch and fibre angle in one ply of a preform;

FIG. 8 a shows part of the inboard portion of a ply containing a singleturn;

FIG. 8 b shows part of the outboard portion of a ply containing a singleturn;

FIG. 9 is a sectional view showing an assembly step of a monolithicconstruction method;

FIG. 10 is a sectional view showing a curing and infusion step of amonolithic construction method;

FIG. 11 is a sectional view of a winglet following the curing step ofFIG. 10 with the inflatable tools removed;

FIG. 12 is a sectional view of a winglet cured using foam tools;

FIG. 13 a is a sectional view showing a first step in the manufacture ofthe leading edge of the winglet;

FIG. 13 b is a sectional view showing a second step in the manufactureof the leading edge of the winglet;

FIG. 14 is a sectional view of the leading edge of the winglet of FIG.13 b with a leading edge skin attached by fasteners; and

FIG. 15 is a sectional view of an alternative winglet leading edge witha tubular front spar.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIGS. 1 a and 1 b show an aircraft 1 with a fuselage 2 carrying a pairof wings 3,4. The aircraft has a horizontal fore/aft axis (labelled X)and a horizontal inboard/outboard axis (labelled Y) normal to thefore/aft axis. Each wing has a winglet and the winglet 5 at the tip ofthe port wing 4 is shown in Figure lc. The port wing 4 comprises a mainwing element 6 with a tip 7, and a winglet 5 attached to the tip. Themain wing element 6 has front and rear spars running along its full spanfrom a root near the fuselage 2 to its tip 7. Only the webs 8,9 of thesespars are shown in FIG. 1 but they also have spar caps which could pointin (towards the other spar) or out. A fuel tank is housed in the mainwing element 6 between the spar webs 8,9.

The winglet 5 has a main (rear) spar 10 and a front spar 11. The mainspar 10 extends from a root 10 a to a tip 10 b which is short of a tip 5a of the winglet 5 so it does not run along the full span of thewinglet. The front spar 11 extends along the full span of the wingletfrom a root 11 a to a tip 11 b. The front spar is C-shaped with a frontspar web 16, a forwardly directed upper front spar cap 17, and aforwardly directed lower front spar cap 18.

As shown in FIG. 2 the main spar 10 of the winglet is tubular (that is,forming a closed cross-sectional shape) with forward and aft main sparwebs 12,13 joined by upper and lower main spar caps 14,15. As shown inFIG. 1 c an upper skin 19 of the winglet is bonded to the upper mainspar cap 14 and the upper front spar cap 17, and a lower skin 20 of thewinglet is bonded to the lower main spar cap 15 and the lower front sparcap 18.

The winglet spars 10,11 have inboard portions which overlap with, andare attached to, the webs 8,9 of the spars in the main wing element 6.The aft web 13 of the winglet main spar 10 is attached to the web 9 byfasteners 21 such as bolts or rivets which pass through holes formed inthe two webs. Similarly the web 16 of the winglet front spar 11 isattached to the web 8, also by fasteners 21 such as bolts or rivetswhich pass through holes drilled in the two webs.

The winglet 6 has three spar webs 12,13,16 (unlike the winglet describedin US 2012/0112005 A1 which has only two) but no transverse webs runningfore and aft and connecting the upper and lower skins (unlike thewinglet described in US 2012/0112005 A1 which has a number of suchtransverse ribs). The closed tubular structure of the main spar 10enables the winglet to handle bending loads more efficiently and meansthat transverse ribs are not required. Whilst the main spar 10 handlesbending loads the front spar 11 stops the winglet from twisting unduly.

The complex geometry of the main spar 10 is shown in detail in FIGS.2-4. The spar 10 extends lengthwise from a root 10 a to a tip 10 b. Ithas a tapered central portion 25 which tapers inwardly towards the tip10 b, a non-tapered inboard portion 26 between the tapered portion andthe root, and a non-tapered outboard portion 27 between the taperedportion and the tip. The aft web of the inboard portion 26 is drilledwith fastener holes 22 for receiving the fasteners 21.

The spar has a centre line 28 shown in dashed lines in FIGS. 3 a-c whichextends lengthwise from the root to the tip passing through thegeometric centre of the spar at each station along its length. Thecentre line 28 is straight in the inboard and outboard portions of thespar, but follows a curved path in the tapered central portion 25. Thiscurved path is doubly curved so it does not lie in a single plane andappears curved from the two orthogonal viewing directions of FIGS. 3 aand 3 b.

FIG. 3 a is a front view of the main spar 10, viewed from the front in adirection parallel to the fore-aft (X) axis of the aircraft. The cantangle of the various parts can be seen in FIGS. 1 a and 3 a. It can beseen from FIG. 1 a that the cant or anhedral angle of the main wingelement (including its spars) is quite small (of the order of 10°) andthe cant or anhedral angle of the centre line 28 of the main spar 10 ofthe winglet increases continuously along the curved path by about 50° asshown in FIG. 3 a.

FIG. 3 b is a plan view of the winglet, viewed vertically from above(like FIG. 1 b) parallel to a vertical Z-axis shown in FIG. 1 a. Theviewing direction of FIG. 3 b is orthogonal to the viewing direction ofFIG. 3 a. The sweep angle of the various parts can be seen in FIGS. 1 aand 3 a. It can be seen from FIG. 1 a that the sweep angle of the mainwing element (including its spars) is quite small whereas the sweepangle of the centre line 28 of the main spar 10 of the winglet increasescontinuously along the curved path by about 15° as shown by FIG. 3 b.

As shown in FIGS. 4 a-d the tapered portion 25 of the winglet main spar10 has a circumference and height which reduce continuously along itslength. Thus the circumference of the spar at station B-B in the inboardportion 26 (FIG. 4 b) is greater than at station C-C in the taperedportion 25 (FIG. 4 c) which in turn is greater than at station D-D inthe outboard portion 27 (FIG. 4 d). Similarly the height of the spar(and of the spar webs) at station B-B (height H1) is greater than atstation C-C (height H2) which in turn is greater than at station D-D(height H3). On the other hand the fore-and-aft width of the taperedportion of the spar (and the width of the spar caps) increases as itextends toward the tip of the spar. Thus the width of the spar atstation B-B (width W1) is less than at station C-C (width W2) which inturn is less than at station D-D (width W3).

The main spar 10 of the winglet comprises a plurality of tubular pliesof braided fibres. A braided dry fibre preform for the main spar 10 isproduced by the braiding apparatus shown in FIGS. 5 and 6. The apparatuscomprises a bobbin braiding ring 30, a braiding ring 31 and a mandrel32. Note that the mandrel is shown in schematic form only in FIG. 5 andin practice will have a complex contoured shape as required to form theinner mould line of the spar 10.

The bobbin braiding ring 30 has a first set of bobbins 35 shown by whitecircles in FIG. 6, and a second set of bobbins 36 shown by blackcircles. Each bobbin carries fibre tows which can be unwound from thebobbin through the braiding ring 31 onto a braid formation point on themandrel 32. Thus as shown in FIG. 6 the first set of bobbins 35 feed afirst set of tows 37 onto the mandrel and the second set of bobbins 36feed a second set of tows 38 onto the mandrel. The first set of bobbinsare rotated clockwise around a winding axis of the bobbin braiding ringat a rotation rate ω1 revolutions per second, and similarly the secondset of bobbins are rotated anti-clockwise around the same winding axisat a rotation rate ω2 revolutions per second (which is normally the sameas ω1). As they rotate around the winding axis the bobbins also followan S-shaped motion 39 so that they weave in and out of the otherbobbins.

Meanwhile the mandrel is traversed in a straight line at a speed S alongthe winding axis so that the first and second sets of tows 37,38 arewound onto the mandrel 32 and become intertwined to form a ply 39 with abraided structure shown in FIG. 5. The process is then repeated (withthe mandrel moving to and fro in opposite directions) to produce apreform with a plurality of tubular braided plies formed one on top ofeach other.

FIG. 7 is a schematic side view of the mandrel 32 and the first set oftows of a single ply formed on the mandrel. As with FIG. 5 the shape ofthe mandrel 32 is schematic and has been simplified relative to the sparof FIG. 1. The first set of tows is wound in a clockwise direction in aseries of turns with a pitch P1, P2 between each adjacent pair of turns.Each tow has a fibre angle θ1, θ2 relative to the winding axis.

As the tow is wound onto the mandrel from left to right in the view ofFIG. 7 the pitch increases and the fibre angle decreases automaticallydue to the reducing mandrel circumference in the tapered portion. Thetraversal speed S of the mandrel is continuously varied from 51 to S2 asthe tow is wound onto the tapered portion of the mandrel. The pitch andfibre angle are both related to the ratio S/ω1 as well as thecircumference of the mandrel, so this change of speed S causes the pitchto increase and the fibre angle to decrease to a greater degree than ifS/ω1 remained constant. If the tow is wound onto the mandrel from leftto right in the view of FIG. 7 (in the direction of decreasingcircumference) then the ratio is increased with time during the winding,and if the tow is wound onto the mandrel from right to left in the viewof FIG. 7 (in the direction of increasing circumference) then the ratiois decreased with time during the winding.

The pitch of the tow continuously varies from P1 in the inboard portionto P2 in the outboard portion, and similarly the fibre anglecontinuously varies from θ1 in the inboard portion to θ2 in the outboardportion. In one example θ1 is +/−45° and θ2 is +/−25° so the fibre anglechanges by 20° in the tapered portion.

The braided ply has a thickness and areal weight both of which arerelated to the pitch and angle of the fibres. The relationship betweenthese parameters is schematically illustrated in FIGS. 8 a and 8 b. FIG.8 a shows part of the inboard portion of a ply which has been cut andunfolded to form a flat rectangular panel with a circumference C1 andlength P1. This panel contains a single turn of a tow of fibres with alength L1 and a fibre angle θ1 of about 45°. The areal weight andthickness of the panel are both proportional to L1/(P1*C1). FIG. 8 bshows part of the outboard portion of a ply which has been cut andunfolded to lie flat to form a flat rectangular panel with acircumference C2 and length P2 (where P1<P2 and C1>C2). This panelcontains a single turn of a tow of fibres with a length L2 and a fibreangle θ2 of about 25°. The areal weight and thickness of the panel areproportional to L2/(P2*C2). In order to achieve constant areal weightand thickness for each ply, the mandrel feed speed S is controlledduring winding so that L1/(P1*C1)=L2/(P2*C2). The mandrel feed speed Sis inversely proportional to the circumference C.

Thus a continuous fibre angle variation is achieved through a gradualsteering of the fibres in the desired direction by variations in mandrelgeometry and mandrel feed speed. The mandrel feed speed is controlled toproduce a ply having constant areal weight and thickness along thelength of the preform. The fibre angle decreases gradually from +/−45°at the inboard portion 26 to +/−25° at the outboard portion 27. As aresult the outboard portion has greater bending stiffness than theinboard portion—bending stiffness being more important at the tip of thespar than at the root of the spar. Conversely the inboard portion hasgreater torsional stiffness and resistance to cracking around near thefastener holes—these properties being more important at the root than atthe tip because there are no fasteners at the tip.

The preform described above is formed with only two set of fibres ineach ply (in other words it is formed by biaxial braiding). Howeveraxial fibres extending lengthwise along the preform may be introduced toform a triaxial braid.

FIGS. 9-11 show a method of manufacturing the winglet 5. In a first stepshown in FIG. 9 upper and lower skin preforms 19 a, 20 a are assembledwith spar preforms 10 a, 11 a and gusset preforms 40 a. The preforms 19a, 20 a, 10 a, 11 a, 40 a are made of dry fibres with no matrix. Thetubular spar preform 10 a is formed using the apparatus and processdescribed above in relation to FIGS. 5 and 6. In the next step shown inFIG. 10, inflatable tools 41-43 are inserted as shown, the structure isplaced in a mould cavity between upper and lower mould tools (not shown)and liquid epoxy resin is injected into the mould cavity to infuse andimpregnate the dry fibre preforms to produce composite spars 10,11,skins 19,20 and gussets 40. Pressure 44 is then applied from theexterior of the winglet by the mould tools, the inflatable tools 41-43are inflated to apply pressure from the inside of the winglet, and theassembly is heated as the pressure 44 is applied to cure the resin inthe various composite parts as well as co-curing the skins to the sparcaps and the gussets.

During the curing process shown in FIG. 10 the webs and caps of the mainspar 10 are compacted against an inflated tool 42 inside the main spar10. Similarly the upper skin, the lower skin, the forward main spar weband the front spar web are compacted against an inflated tool 43 betweenthe main spar 10 and the front spar. Similarly the aft parts of theskins are compacted against an inflated tool 41 aft of the main spar 10.After cure, the inflatable tools 41-43 are deflated and removed from theroot of the winglet, leaving the cured structure shown in FIG. 11.

Alternatively the inflatable and removable tools shown in FIG. 10 can bereplaced by foam curing tools 50 shown in FIG. 12. These foam tools canbe left inside the finished article instead of being removed.

FIGS. 13 a and 13 b show two steps in the manufacture of the front spar11. First a tubular front spar preform 11 b is formed (by braiding orany other method such as tape laying or fibre placement). The tubularfront spar preform 11 b has forward and aft webs 16,51 joined by upperand lower front spar caps. The tubular front spar preform 11 b is fittedwith an inflatable tool 52 and then infused and cured along with theother parts of the winglet in the process shown in FIG. 10 or 12. Duringthis curing process both webs 16,51 and both caps of the tubular frontspar are compacted against the inflated tool 52. After cure is complete,the front half 53 of the tubular front spar is cut away as shown in FIG.13 b and removed along with the deflated tool 52, leaving the C-sectionfront spar 11 as shown.

Finally a leading edge skin is attached to the upper and lower spar capsby fasteners as shown in FIG. 14. The leading edge skin comprises anupper leading edge skin panel 60 attached at its aft edge to the upperspar cap 17 by fasteners 65, a lower leading edge skin panel 61 attachedat its aft edge to the lower spar cap 18 by fasteners 66; and a curvedD-nose skin panel 62 connecting the upper and lower skin panels. Theskin panels 60-62 may be separate parts or they may be formed togetheras a single integral piece. The upper and lower skin panels 60,61 lieflush with the upper and lower skins 19,20.

Alternatively the leading edge skin may be co-cured to the upper andlower spar caps without fasteners as shown in FIG. 15 A fifth inflatabletool 70 is provided as shown, and during the curing process the forwardpart of the leading edge skin 60-62 is compacted and cured against thistool 70. The aft parts of the leading edge skin panels 60,61 arecompacted against the upper and lower caps of the tubular front spar towhich they become co-cured. The tools 52,70 are then removed but thefront half of the tubular front spar is not cut away. In the case ofFIG. 15 both the main and front spars of the winglet are tubular in thefinished article.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

1. A winglet comprising a tubular main spar with forward and aft mainspar webs joined by upper and lower main spar caps; a front spar with afront spar web, an upper front spar cap, and a lower front spar cap; anupper skin joined to the upper main spar cap and the upper front sparcap; and a lower skin joined to the lower main spar cap and the lowerfront spar cap.
 2. The winglet of claim 1 wherein the upper front sparcap extends forwards away from the main spar, and the lower front sparcap extends forwards away from the main spar.
 3. The winglet of claim 1wherein the main spar has a tapered portion in which a height of themain spar reduces and a fore-and-aft width of the main spar increases asit extends toward a tip of the spar
 4. The winglet of claim 1 whereinthe main spar has a centre line which extends lengthwise from a root ofthe winglet to a tip of the winglet, and at least part of the centreline follows a curved path.
 5. The winglet of claim 4 wherein the curvedpath does not lie in a single plane so that a cant angle and a sweepangle of the main spar both change continuously along the curved path.6. The winglet of claim 1 wherein the main spar comprises a plurality oftubular plies of braided fibres impregnated with a matrix.
 7. Thewinglet of claim 1 wherein the front spar is tubular with forward andaft front spar webs joined by the upper and lower front spar caps. 8.The winglet of claim 1 wherein the upper skin is bonded to the uppermain spar cap and the upper front spar cap; and the lower skin is bondedto the lower main spar cap and the lower front spar cap.
 9. The wingletof claim 1 further comprising a leading edge skin joined to the upperand lower front spar caps.
 10. The winglet of claim 1 wherein theleading edge skin is bonded to the upper and lower front spar caps. 11.An aircraft wing comprising a main wing element with a tip; and awinglet according to claim 1 attached to the tip of the main wingelement.
 12. The wing of claim 11 wherein the main wing elementcomprises a rear spar which is attached to the tubular main spar of thewinglet; and a front spar which is attached to the front spar of thewinglet.
 13. The wing of claim 12 wherein the tubular main spar of thewinglet is canted up or down relative to the rear spar of the main wingelement.
 14. The wing of claim 12 wherein the tubular main spar of thewinglet is swept forward or aft relative to the rear spar of the mainwing element.
 15. The wing of claim 12 wherein the spars of the winglethave inboard portions which overlap with, and are attached to, the sparsof the main wing element by fasteners.
 16. A method of manufacturing thewinglet of claim 1, the method comprising co-curing the upper skin tothe upper main spar cap and the upper front spar cap; and co-curing thelower skin to the lower main spar cap and the lower front spar cap. 17.The method of claim 16 further comprising during the co-curing:compacting the webs and caps of the main spar against a first toolinside the main spar; compacting the upper skin, the lower skin, theforward main spar web, and the front spar web against a second toolbetween the main spar and the front spar; and compacting the front sparweb and the upper and lower front spar caps against a third tool infront of the front spar web.
 18. The method of claim 16 furthercomprising joining a leading edge skin to the upper and lower front sparcaps.
 19. The method of claim 18 wherein the leading edge skin isco-cured to the upper and lower front spar caps.
 20. The method of claim19 wherein the front spar is tubular with forward and aft front sparwebs joined by the upper and lower front spar caps, and the methodfurther comprises during the co-curing compacting both webs and bothcaps of the front spar against the third tool which is inside thetubular front spar; and compacting the leading edge skin and the forwardweb of the front spar against a fourth tool in front of the tubularfront spar.
 21. The method of claim 19 further comprising providing atubular front spar with forward and aft front spar webs joined by theupper and lower front spar caps, and the method further comprises duringthe co-curing compacting both webs and both caps of the front sparagainst the third tool which is inside the tubular front spar; and afterthe co-curing cutting away the forward front spar web from the aft frontspar web and removing it along with the third tool; and joining theleading edge skin to the upper and lower front spar caps with fasteners.22. The method of claim 17 further comprising removing the tools afterthe co-curing.